The overall goal of this project is develop tandem mass spectrometry for newborn screening of the subset of lysosomal storage diseases for which treatment options exist or are being developed. A punch from a dried blood spot on a newborn screening card is used as a source of lysosomal enzymes, and this is incubated with a collection of substrates in a suitable buffer to allow formation of products. The latter are quantified by tandem mass spectrometry with the aid of a set of internal standards. We have developed a tandem mass spectrometry multiplex assay of six lysosomal enzymes. This 6-plex assay will be piloted in the WA state newborn screening laboratory on n=100,000 dried blood spots from random newborns. The next phase is to develop highly sensitive reagents to assay 6 sulfatase enzymes relevant to 6 additional lysosomal storage diseases. We will also develop tandem mass spectrometry assays for tripeptidyl protease I (deficiency causes neuronal ceroid lipofuscinosis 2), lysosomal acid lipase (deficiency causes Wolman disease and cholesterol-ester storage disease). We will then develop a multiplex method for newborn screening of all 13 enzymes using the minimum number of dried blood spot punches and assay buffers. Once this multiplex assay has been developed, we will carry out a second pilot study in the WA state newborn screening lab on n=100,000 dried blood spots from random newborns. We will also carry out exome DNA sequencing using next-generation sequencing on dried blood spots that give an enzyme activity below the cut-off value. This data will allow us to determine the positive predictive values and false positive rates for the screening assay. These pilot studies will explore the feasibility of newborn screening of treatable lysosomal storage diseases.